fix(agent): reject send tool when targeting the same conversation

Pass replyTarget through the full pipeline (ChatRequest → gateway
identity → agent headers → MCP session) so the send tool can detect
when the destination matches the current conversation and return an
error guiding the agent to reply directly instead.

apps/agent/src/models.ts

@@ -31,6 +31,7 @@ export const IdentityContextModel = z.object({
   channelIdentityId: z.string().min(1, 'Channel identity ID is required'),
   displayName: z.string().min(1, 'Display name is required'),
   currentPlatform: z.string().optional(),
+  replyTarget: z.string().optional(),
   conversationType: z.string().optional(),
   sessionToken: z.string().optional(),
 })